Chimney and Industrial Stack Structural Design
A tall chimney or process stack is one of the few structures where the entire lateral system is a single hollow shell − there is no redundant frame behind it, no secondary member to pick up load if the primary one is overstressed. That means every check on a stack, from wind pressure to thermal gradient through the wall, is effectively a check on one part, and a designer has to size the shell, the liner, and the foundation with the understanding that there is nowhere else for the load to go.
Shell Design and Ovaling
Steel stacks are usually designed as cylindrical or tapered shells, and beyond the basic bending and axial stress from wind and self-weight, a slender unstiffened cylinder is vulnerable to ovaling − a local cross-sectional distortion driven by the same vortex shedding that produces across-wind vibration, covered in more depth in our piece on vortex shedding and wind-induced vibration. Ovaling shows up as a breathing-mode deformation of the shell wall at a frequency distinct from the stack's overall bending frequency, and it is controlled with stiffening rings spaced up the height rather than by adding wall thickness, since thickness addresses bending stress but does little for a local shell-buckling mode. Concrete chimneys, built either as slip-formed cast-in-place shells or increasingly rare today compared to steel, carry similar wind and vibration checks but add reinforcement detailing for a section that is in tension on the windward face under combined wind and thermal loading, a load case that does not exist in most building design.
A stack's own flue gas creates a temperature gradient through the shell wall − hot on the inside face, ambient on the outside − and that gradient produces a self-equilibrating thermal stress across the wall thickness independent of any external load. For steel stacks with a refractory or insulating liner this effect is manageable, but an unlined steel stack running hot flue gas directly against bare steel can see thermal stress become a controlling design case rather than a secondary one.
Liner Support and Annular Space
Many stacks separate the load-carrying outer shell from an inner liner (brick, refractory, or corrosion-resistant metal) that actually contacts the flue gas, with the two connected only at discrete support points rather than continuously. This split matters structurally because the liner and shell expand thermally at different rates and different times − the liner heats up on startup faster than the outer shell − and the support brackets between them have to accommodate that differential movement the same way a curtain wall anchor accommodates movement between facade and frame, without transferring the liner's thermal expansion into the outer shell as an unintended load. Corrosion of liner supports from condensing acidic flue gas is one of the more common in-service failure modes on older stacks, which is why liner brackets are usually the first thing inspected on a stack nearing the end of its liner's service life.
Foundation Overturning
A stack's foundation resists almost pure overturning moment from wind rather than the mixed gravity-and-lateral demand typical of a building footing, because a stack has comparatively little dead load relative to its height and wind moment arm. That shifts the governing foundation check toward soil bearing pressure at the leeward edge of the footing and, for stacks on piles, net uplift on the windward piles rather than the compression-dominated checks common to the foundation types covered in our foundation types overview. Anchor bolts connecting a steel stack's base plate to the foundation see the same wind moment translated into cyclic tension and compression as the stack sways, and because there are relatively few anchor bolts around a stack's circumference compared to the number of connections in a redundant building frame, each bolt group carries a larger share of the total overturning demand and gets correspondingly more design attention.
Stack height is also driven by a non-structural constraint worth knowing: dispersion requirements for pollutants can push a stack taller than structural loading alone would justify, since regulators evaluate whether emissions clear surrounding buildings and terrain features before they disperse, a topic covered in EPA guidance on good engineering practice stack height (epa.gov) that structural engineers coordinate with rather than control directly, even though the added height feeds straight back into the wind and dynamic checks described above.